ARM: How to Retain Data in Object File

Information in this article applies to:

• Keil MDK 4.10 and later

QUESTION

How do I keep data in my output image regardless of whether they are referenced or not like a constant version number or checksum?

ANSWER

Append the USED attribute to your constant. Please refer to the example below:

        unsigned const int checksum __attribute__((used)) = 0x12345678;

The same attribute can also be used for static functions to avoid removal from the object file.

NOTE

Static variables marked as used are emitted to a single section, in the order they are declared. Please refer to the following example:

  static char a1;   /* at offset 0 */
  static int  b1;   /* at offset 4 */
  static char c1;   /* this will be placed at offset 1 to exploit the gap  */

  /* these ones will not only be kept but also not reordered */
  static char a2 __attribute__((used));   /* at offset 0 */
  static int  b2 __attribute__((used));   /* at offset 4 */
  static char c2 __attribute__((used));   /* at offset 8 */

You can specify the section that variables are placed in using __attribute__((section)).

MORE INFORMATION

• Refer to __attribute__((used)) variable attribute in the Compiler User Guide.

SEE ALSO

• Refer to __attribute__((section("name"))) variable attribut in the Compiler User Guide.

Last Reviewed: Tuesday, December 22, 2020